BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to cavities and channel add/drop filters employing
photonic crystals, and in particular to improvements in the characteristics of cavities
and channel add/drop filters based on two-dimensional photonic crystals.
[0002] It should be understood that in the present specification, the significance of the
term "light" is meant to also include electromagnetic waves that relative to visible
light are of longer as well as shorter wavelength.
Description of the Background Art
[0003] Along with advances in wavelength division multiplexed (WDM) optical communication
systems in recent years, the importance of ultrasmall add/drop filters and channel
filters in which enlarged capacity is being targeted is on the rise. In this area,
then, attempts are being made to develop extraordinarily small-scale optical add/drop
filters by employing photonic crystals. In particular, with photonic crystals novel
optical properties can be realized by exploiting artificial periodic structures in
which a crystal-lattice-like periodic refractive index distribution is artificially
imparted within the parent material.
[0004] One important feature of photonic crystals is the presence of photonic bandgaps.
With photonic crystals having three-dimensional refractive index periodicity (3D photonic
crystals), perfect bandgaps in which the transmission of light is prohibited in every
direction can be formed. Among the possibilities with these crystals are the local
confinement of light, control of spontaneous emission, and formation of waveguides
by the introduction of line defects, wherein the realization of ultrasmall photonic
integrated circuits can be anticipated.
[0005] Meanwhile, studies into uses for photonic crystals having a two-dimensional periodic
refractive-index structure (2D photonic crystals), are flourishing because the crystals
can be manufactured comparatively easily. A periodic refractive-index structure in
2D photonic crystals can be formed by, for example, arranging in a square-lattice
or triangular-lattice geometry air rods perforating a high-refractive-index plate
material (usually termed a "slab"). Alternatively the structure can be formed within
a low-index material by arranging, in a 2D-lattice geometry within it, posts made
of a high-refractive-index material. Photonic bandgaps can be produced from such periodic
refractive-index structures, enabling the transmission of light traveling in an in-plane
direction (direction parallel to both the principal faces of the slab) to be controlled.
Waveguides, for instance, may be created by introducing line defects into a periodic
refractive-index structure. (See, for example,
Physical Review B, Vol. 62, 2000, pp. 4488-4492.)
[0006] Fig. 12 illustrates, in a schematic oblique view, a channel add/drop filter disclosed
in Japanese Unexamined Pat. App. Pub. No. 2001-272555. (In the drawings in the present
application, identical reference marks indicate identical or equivalent parts.) The
channel add/drop filter in Fig. 12 exploits a 2D photonic crystal having, configured
within a slab 1, cylindrical through-holes 2 of identical diameter (ordinarily occupied
by air) formed at the vertices of a 2D triangular lattice. In a 2D photonic crystal
of this sort, light is prohibited from propagating in an in-plane direction within
the slab 1 by a bandgap, and in the direction normal to the plane (direction orthogonal
to the two principal faces of the slab) is confined due to total internal reflection
occurring at the interface with the low-refractive-index clad (air, for example).
[0007] The photonic crystal in Fig. 12 contains a waveguide 3 consisting of a straight line
defect. This straight-line defect 3 includes a rectilinearly ranging plurality of
lattice points adjoining each other, with the through-holes 2 missing in these lattice
points. With light being able to propagate through defects in the 2D photonic crystal,
the straight-line defect can be employed as a linear waveguide. With linear waveguides,
the spectrum of wavelengths in which light can be transmitted at low loss is comparatively
broad; consequently light in a wide range of wavelength containing signals in a plurality
of channels may be propagated through them.
[0008] It will be appreciated that the width of straight-line defects as waveguides can
be altered variously in accordance with the requested characteristics. The most typical
waveguide is obtained, as described above, by leaving through-holes missing in one
row in lattice-point line. Nevertheless, waveguides can also be created by leaving
through-holes missing in a plurality of neighboring rows in the lattice-point lines.
Moreover, a waveguide is not limited in width to integral multiples of the lattice
constant, but may have an arbitrary width. For example, it is possible to create a
waveguide having a width of choice by relatively displacing the lattice on either
side of a linear waveguide to the distance of choice.
[0009] The photonic crystal set out in Fig. 12 also contains a cavity 4 consisting of a
point defect. The point defect 4 contains a single lattice point, and through that
lattice point a through-hole that is of large diameter as compared with the other
lattice points is formed. A defect in this way containing a relatively large-diameter
through-hole is generally termed an acceptor-type point defect. On the other hand,
a defect in which a through-hole is missing in a lattice point is generally termed
a donor-type point defect. The cavity 4 is disposed adjacent the waveguide 3, within
a range in which they can exert on each other an electromagnetically reciprocal effect.
[0010] In a 2D photonic crystal such as that illustrated in Fig. 12, if light 5 containing
a plurality of wavelength ranges (λ
1, λ
2, ... λ
i, ...) is introduced into the waveguide 3, light that has the specific wavelength
corresponding to the resonant frequency of the cavity 4 is trapped in the cavity and
while resonating in the interior of the point defect, light 6 of wavelength λ
i is emitted in the normal direction, in which the Qfactor originating in the finite
thickness of the slab 1 is small. This means that the photonic crystal in Fig. 12
can be employed as a channel drop filter. Conversely, by shining light into the point
defect 4, in the direction normal to the slab 1, light of wavelength λ
i that resonates within the cavity 4 can be introduced into the waveguide 3. This means
that the photonic crystal in Fig. 12 can also be employed as a channel add filter.
It will be appreciated that the transfer of light between either the waveguide 3 or
the cavity 4 and the exterior can be made to take place by proximately disposing an
optical fiber or an optoelectronic transducer in the vicinity of the waveguide end
faces or the vicinity of the cavity. Of course, in that case a collimating lens (collimator)
may be inserted in between either the waveguide end face or the cavity, and the optical-fiber
end face or the optoelectronic transducer.
[0011] In a an optical add/drop filter such as that illustrated in Fig. 12, by appropriately
configuring the spacing between the waveguide 3 consisting of the line defect and
the cavity 4 consisting of the point defect, the ratio of optical intensities in a
specific wavelength that is transferred between the waveguide and the cavity can be
controlled. Also in Fig. 12, since no asymmetry is introduced with respect to the
point defect 4 in the direction normal to the slab 1, light is output in both vertical
directions from the point defect 4; but it is possible to make the output of light
be in only one or the other vertical direction by introducing asymmetry in the point
defect 4 in the plane-normal direction. An example of a mechanism that can be utilized
to introduce this sort of asymmetry is a method in which the diameter of the point
defect 4, which is round in section, is made to vary continuously or discontinuously
along the thickness of the slab. With further regard to Fig. 12, although the channel
add/drop filter in the figure contains only a single cavity, it will be readily understood
that by disposing along the waveguide a plurality of cavities differing from one another
in resonant wavelength, optical signals in a plurality of channels can be added/dropped.
[0012] With the
Q factor of a cavity employing an acceptor-type point defect such as disclosed in Japanese
Unexamined Pat. App. Pub. No. 2001-272555 being around 500, the full width at half-maximum
(FWHM) in the peak-wavelength-including light output from a cavity of this sort is
around 3 nm.
[0013] However, using multi-channel signals for WDM communications at about 100 GHz with
a wavelength-peak spacing of approximately 0.8 nm is being investigated. This means
that with a cavity such as disclosed in Unexamined Pat. App. Pub. No. 2001-272555,
the largeness of the
Q factor is insufficient, and with the 3-nm FWHM, the cavity is totally inadequate
for separating from one another multi-channel signals whose peak-wavelength spacing
is approximately 0.8 nm. In short, there is a need to raise the
Q factor of cavities employing 2D photonic crystals, to reduce the FWHM of the peak-wavelength
spectra they output.
SUMMARY OF THE INVENTION
[0014] A principal object of the present invention, in view of the situation with the conventional
technology, is to afford a high-
Q cavity within a 2D photonic crystal, and furthermore to combine such a cavity with
a waveguide to make available a channel add/drop filter having high wavelength resolution.
[0015] A cavity made from a point defect within a two-dimensional photonic crystal in accordance
with the present invention―the 2D photonic crystal being configured by an arrangement,
in a two-dimensional lattice of points defined in a slab, of low-refractive-index
substances having a low refractive index relative to the slab and being identical
dimension and shape―is characterized in that the point defect contains a plurality
of three or more lattice points that neighbor one another, and in these lattice points
no low-refractive-index substances are arranged, and in that the low-refractive-index
substance that should be arranged to correspond to at least one of the lattice points
nearest the point defect is displaced by a predetermined distance from that lattice
point.
[0016] Here, the low-refractive-index substance that would otherwise be arranged to correspond
to at least one of the lattice points secondarily adjacent the point defect may be
displaced by a predetermined amount from that lattice point. Preferably, furthermore,
the point defect contains six or fewer of the lattice points. The wavelength of light
that resonates in the cavity is adjustable in dependency upon the dimension and shape
of the point defect, or may be adjusted by changing the lattice constant of the photonic
crystal. It is preferable that the point defect contain the plurality of lattice points
lined in a line segment.
[0017] The low-refractive-index substances can be filled into columns perforating the slab.
The points in the two-dimensional lattice preferably are arrayed in a triangular lattice.
The slab preferably has a refractive index of 2.0 or greater.
[0018] A channel add/drop filter in accordance with the present invention, including one
or more cavities as in the foregoing, includes one or more waveguides made from a
line defect within the two-dimensional photonic crystal, and is characterized in that
the cavity is disposed adjacent the waveguide, within a separation in which between
them an electromagnetically reciprocal effect is produced. By containing a plurality
of cavities that differ from one another in resonant frequency, a channel add/drop
filter of this sort can function as a channel add/drop filter for multi-channel optical
communications.
[0019] From the following detailed description in conjunction with the accompanying drawings,
the foregoing and other objects, features, aspects and advantages of the present invention
will become readily apparent to those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
Fig. 1 is a schematic plan view for explaining principal features in one example of
a cavity in a photonic crystal according to the present invention;
Fig. 2 is a picture, in a simulation pertaining to one example of a cavity within
a 2D photonic crystal, showing the radiation pattern of light from the cavity, seen
in the direction normal to the slab;
Fig. 3 is a picture, in a simulation pertaining to one example of a cavity according
to the present invention, showing the radiation pattern of light from the cavity,
seen in the direction normal to the slab;
Fig. 4 is a picture, in a simulation pertaining to another example of a cavity according
to the present invention, showing the radiation pattern of light from the cavity,
seen in the direction normal to the slab;
Fig. 5 is a graph that plots the relationship between displacement n in the Γ -J direction and Q factor, for a point defect as illustrated in Fig. 1;
Fig. 6 is a graph that plots the power ratio of side lobes with respect to the main
emission beam from a cavity, in relationship to displacement n;
Fig. 7 shows the radiation pattern of light from a cavity, seen in the direction normal
to the slab in a simulation pertaining to yet another example of a cavity according
to the present invention;
Fig. 8 is a plan view schematically illustrating a situation in which not only at
least one of through-holes corresponding to the lattice points nearest a point defect,
but also at least one of through-holes corresponding to the secondarily adjacent lattice
points, is displaced by a predetermined distance from its corresponding lattice point;
Fig. 9 is a scanning electron micrograph (SEM) showing a channel add/drop filter in
a 2D photonic crystal actually fabricated by the present invention;
Fig. 10 is a graph that plots the relationship between wavelength and intensity of
light emitted from a cavity in the direction normal to the slab, in a case where light
including a variety of wavelengths was introduced into the waveguide in Fig. 9;
Fig. 11 is a schematic oblique view illustrating a channel add/drop filter in another
example of an embodiment of the present invention;
Fig. 12 is a schematic oblique view illustrating a channel add/drop filter employed
in a 2D photonic crystal according to the prior art; and
Figs. 13A and B are schematic plan views representing examples of donor-type point
defects that contain a plurality of lattice points, in a 2D photonic crystal.
DETAILED DESCRIPTION OF THE INVENTION
[0021] Initially the present inventors looked into the characteristics, within a 2D photonic
crystal, not of a cavity consisting of an acceptor-type defect as in Fig. 12, but
of a cavity consisting of a donor-type point defect. As described earlier, donor-type
defects contain one or more lattice points, and through-holes are missing in those
lattice points.
[0022] What has chiefly been studied to date are point defects containing only a single
lattice point, from the perspectives that owing to their structural simplicity they
are easily analyzed electromagnetically and that they are of minimal size. This has
meant that with donor types as well, point defects that contain a plurality of lattice
points have not to date been studied extensively. Given the circumstances, then, the
present inventors investigated the characteristics of donor-type point defects that
contain a plurality of lattice points.
[0023] Fig. 13 is a schematic plan view representing a portion of a 2D photonic crystal
including a donor-type point defect that contains a plurality of lattice points. In
this 2D photonic crystal, through-holes 2 are provided at the vertices of a triangular
lattice configured within a slab 1. Point defect 4 in Fig. 13A contains three lattice
points neighboring one another in line-segment form, with no through-holes 2 being
furnished in these lattice points. Meanwhile, point defect 4 in Fig. 13B contains
three lattice points neighboring one another in a triangular geometry, with no through-holes
2 being furnished in these lattice points. In other words, the point defect 4 can
be formed to contain a plurality of lattice points neighboring one another one-dimensionally,
or may be formed to contain a plurality of lattice points neighboring one another
two-dimensionally.
[0024] Using the widely known finite-difference time-domain (FDTD) method (see Japanese
Unexamined Pat. App. Pub. No. 2001-272555), the present inventors carried out electromagnetic
analyses on donor-type defects containing a plurality of lattice points, wherein they
found that compared with cavities consisting of donor-type point defects that contain
one or two lattice points, high
Q values are obtained with cavities consisting of donor-type point defects that contain
three or more lattice points. Nevertheless, if the number of lattice points that are
contained in the point defect is too large, the number of resonant modes will be undesirably
many, thus the number of lattice points preferably is six or fewer.
[0025] For example, with a cavity as is represented in Fig. 13A, in the simple base structure,
Q = 5200, and when compounded with a waveguide the filter is able to yield a
Q factor of about 2600, with the FWHM of the light output from the cavity being approximately
0.6 nm. Still, taking into account crosstalk in WDM optical communication that employs
multi-channel signals at about 100 GHz with a wavelength-peak spacing of approximately
0.8 nm, further improvement in
Q factor is to be desired.
[0026] Fig. 1 is a schematic plan view for explaining principal features in one example
of a cavity as defined by the present invention. A two-dimensional lattice of triangular
vertices is defined in this 2D photonic crystal in Fig. 1, and round tubular through-holes
2 of identical shape are formed at the lattice points. The spacing between adjacent-most
lattice points in (the lattice constant of) the triangular lattice is indicated by
a. The donor-type point defect illustrated in Fig. 1 contains three lattice points
neighboring one another and ranging in a line-segment formation; the through-holes
2 are missing in these lattice points.
[0027] A principal feature with the donor-type point defect according to the present invention
is that at least one of the through-holes 2 nearest the point defect is formed displaced
by a predetermined distance from its corresponding lattice point. In Fig. 1 the Γ
-
X and Γ -
J axes, which are at right angles to each other, indicate the directions in which the
through-hole 2 is set apart from its corresponding lattice point. In Fig. 1 also,
the arrows labeled with reference marks
l, m and
n indicate the directions in which the through-holes 2 formed corresponding to the
lattice points nearest the point defect are displaced from those lattice points. It
will be appreciated that inasmuch as the displacement directions in Fig. 1 are merely
for illustration, the through-holes 2 may of course be displaced in any direction
of choice.
[0028] Hereinafter the state in which the through-holes 2 nearest the point defect are displaced
from the original lattice points to which they correspond is expressed as "displacement
=
(l, m, n)." For example, the expression "displacement = (0.1
a, 0.2
a, 0.3
a)" means that the through-holes corresponding to the arrows labeled with reference
mark
l are displaced from their corresponding lattice points by the distance 0.1
a, likewise means that the through-holes corresponding to the arrows labeled with reference
mark
m have been displaced from their corresponding lattice points by the distance 0.2
a, and that the through-holes corresponding to the arrows labeled with reference mark
n have been displaced from their corresponding lattice points by the distance 0.3
a.
[0029] The
Q factor and electric field pattern (radiation pattern) for a cavity made from a donor-type
point defect 4 as is illustrated in Fig. 1 were simulated by the FDTD method. The
simulation parameters were configured by selecting silicon for the slab 1; and setting
approximately 1.55 µm, which is generally used in optical communications, for the
wavelength λ; 0.42 µm for the lattice constant
a; 0.6
a for the slab 1 thickness; and 0.29
a for the sectional radius of the through-holes 2.
[0030] For a case where
(l, m, n) = (0, 0, 0) in the simulation under these conditions, a
Q factor of 5200 was obtained; Fig. 2 shows the radiation pattern of light from the
cavity 4 in this case, seen in the direction normal to the slab 1. For a case where
(l, m, n) = (0, 0, 0.15
a) in a similar simulation, a
Q factor of 43,000 was obtained; Fig. 3 shows the radiation pattern of light from the
cavity 4 in this case.
[0031] As will be understood from these simulations, in a donor-type point defect that contains
three lattice points neighboring one another in a line-segment formation, displacing
the through-hole neighboring both ends of the line segment a distance of 0.15
a from its corresponding lattice point dramatically raises the
Q factor from 5200 to 43,000 and meanwhile, as will be understood from a comparison
between Figs. 2 and 3, reduces the radiation angle of the light.
[0032] In a further case, in which the displacement n was made greater by making (
l,
m, n) = (0, 0, 0.20
a), a still higher
Q = 100,000 was obtained; Fig. 4 shows the radiation pattern of light from the cavity
in this case. Compared with Fig. 3, in Fig. 4 the radiation angle of the light is
large, and side lobes (secondary beams) above and below the main emission beam in
the center in Fig. 4 are conspicuous. This means that along with an enlargement of
the distance by which the through-holes 2 nearest the point defect 4 are displaced
from their corresponding lattice points, the
Q also trends to an enlarged value, but considered in terms of the radiation angle
of the light from the cavity 4, the displacement should not necessarily be large.
[0033] Reference is made to Fig. 5, which is a graph that plots the relationship between
displacement
n in the Γ
-J direction and
Q factor, for a point defect as illustrated in Fig. 1. In the graph, the horizontal
axis indicates displacement
n scaled by the lattice constant
a, while the vertical axis expresses
Q factor. From Fig. 5 it will be understood that there is a limit to how far
Q factor is enlarged by increasing the displacement
n. Specifically, as the displacement
n is increased up to 0.20
a the
Q factor increases exponentially also, reaching a maximum value of 100,000; but if
the displacement
n is increased further the
Q factor on the contrary decreases sharply.
[0034] In the Fig. 6 graph, the relationship between the displacement
n and the power ratio of side lobes, such as can be distinctly seen in Fig. 4, to the
main emission beam is shown. In the graph, the horizontal axis indicates displacement
n scaled by the lattice constant
a, while the vertical axis expresses the ratio of side-lobe emission power to the main
emission beam. In Fig. 6 it is evident that the emission-beam radiation angle containing
the side lobes is smallest in the case where the displacement
n is 0.15
a, while it is greatest in the case where the displacement
n is 0.25
a.
[0035] A high
Q = 11,900 is also obtained in a case where the displacements are (
l, m, n) = (0.11
a, 0.11
a, 0), compared with the
(l, m, n) = (0, 0, 0) case where the through-holes are not displaced from the lattice points;
Fig. 7 shows the radiation pattern of light from the cavity in this case. As will
be understood from a comparison with the
(l, m, n) = (0, 0, 0) case in Fig. 3, in Fig. 7 the radiation angle of the light emission is
smaller.
[0036] Reference is now made to Fig. 8, a schematic plan view that, while resembling Fig.
1, diagrammatically illustrates a case where not only at least one of the through-holes
2 corresponding to the lattice points nearest the point defect 4, but also at least
one of the through-holes 2 corresponding to the secondarily adjacent lattice points,
is displaced by a predetermined distance from its corresponding lattice point. Although
what is most effective improving the
Q factor of the cavity is as described above displacing through-holes corresponding
to the lattice points nearest the point defect 4 by a predetermined distance from
its corresponding lattice point, additionally displacing through-holes 2 corresponding
to the secondarily adjacent lattice points by a predetermined distance from its corresponding
lattice point also produces an effect of bettering the
Q factor.
[0037] Reference is now made to Fig. 9, a scanning electron micrograph (SEM) showing a portion
of an actually fabricated 2D photonic crystal. Configuration parameters for the 2D
photonic crystal―including the substance for the slab 1, the two-dimensional lattice
constant, the through-hole 2 diameter, and the number and arrangement of the lattice
points that the point defect 4 contains are likewise as was the case in the simulation
set forth above, while the displacements were set to
(l, m, n) = (0, 0, 0.15
a).
[0038] Electron-beam lithography and reactive ion etching (see Japanese Unexamined Pat.
App. Pub. No. 2001-272555) were employed to fabricate the photonic crystal of Fig.
9 and to contain, in addition to the point defect 4, a straight waveguide 3. This
meant that light of a predetermined wavelength could be transferred between the cavity
constituted by the point defect 4, and the straight waveguide 3, allowing the device
to function as a channel add/drop filter.
[0039] In the Fig. 10 graph, the relationship between wavelength and intensity of light
emitted from the cavity 4 in the direction normal to the slab 1, in a case where light
including a variety of wavelengths was actually introduced into the waveguide 3 in
Fig. 9. In particular, the horizontal axis in the graph expresses wavelength (nm),
and the vertical axis expresses intensity of light (a.u.: arbitrary units). As is
evident from Fig. 10, the cavity 4 included in the channel add/drop filter of Fig.
9 extracted from the wavelengths of light introduced into the waveguide 3 light having
a peak wavelength of approximately 1578.2 nm, emitted at a full width at half maximum
(FWHM) of approximately 0.045 nm, and had a high
Q factor―as anticipated by the above-described simulation―of about 35,100. It will
thus be understood that as provided for by the present invention, a channel add/drop
filter having a high wavelength resolution can be achieved.
[0040] It should be understood that although in the channel add/drop filter of Fig. 9 only
one cavity is disposed proximate the one waveguide, a multi-channel add/drop filter
that can handle optical communications in a plurality of channels differing from one
another in wavelength can of course be created by disposing in proximity along a single
waveguide a plurality of cavities differing from one another in resonant frequency.
Also, by disposing the end face of an optical fiber to confront the cavity 4 proximately,
the light emitted from the cavity 4 in the direction normal to the slab 1 can be introduced
into the optical fiber. Furthermore, by disposing an optoelectronic transducer to
confront the cavity 4 proximately intensity modulations in the light from the cavity
can be received. It will be appreciated by those skilled in the art that a collimating
lens (collimator) may be inserted in between the cavity 4 and either the optical-fiber
end face or the optoelectronic transducer.
[0041] Reference is now made to Fig. 11, schematically illustrating in an oblique view a
channel add/drop filter in another example of an embodiment of the present invention.
Although the channel add/drop filter of Fig. 11 resembles that of Fig. 9, in Fig.
11 a cavity 4 is disposed adjacent a first straight waveguide 3a and further, a second
waveguide 3b is disposed adjacent the cavity 4. In this instance, as described earlier
an optical signal of a specific wavelength can be extracted within the cavity 4 from
optical signals introduced into the first waveguide 3a, but with the second waveguide
3b being disposed adjacent the cavity 4 the extracted optical signal is introduced
from the cavity 4, not in the plane-normal of, but into the second waveguide 3b in,
the slab 1. This means that in a channel add/drop filter employing a 2D photonic crystal,
an optical signal of a given wavelength among optical signals that propagate through
one waveguide can be selectively guided into another waveguide.
[0042] A material whose refractive index is large is desirable as the slab 1 for the photonic
crystal, inasmuch as it must confine light along its thickness. In the embodiments
described above, an Si (silicon) slab was utilized, but materials other than silicon
that may be utilized include: Group IV semiconductors such as Ge, Sn, C and SiC; Group
III-V semiconductor compounds such as GaAs, InP, GaN, GaP, AIP, AIAs, GaSb, InAs,
ALSb, InSb, InGaAsP and AlGaAs; Group II-VI semiconductor compounds such as ZnS, CdS,
ZnSe, HgS, MnSe, CdSe, ZnTe, MnTe, CdTe and HgTe; oxides such as SiO
2, Al
2O
3, and TiO
2; silicon nitride; various glass of all sorts, such as soda-lime glass; as well as
organic substances such as Alq3 (C
27H
18AlN
3O
3. In situations where amplification of optical signals in photonic crystals constituted
from these slabs is desired they may be doped with Er.
[0043] It is preferable that the refractive index of the slab 1 be, specifically, greater
than air―2.0 or greater, with 3.0 or greater being more preferable. It will be appreciated
by those skilled in the art that while air is present within the through-holes 2 in
the embodiments described above, a substance of low refractive index relative to the
slab 1 may of course be filled into the through-holes 2. A substance such as conducting
polythiophene, for example, may be utilized as the low-refractive-index material.
Furthermore, the two-dimensional lattice configured within the slab 1 is not limited
to being a triangular lattice but can be configured as any other regular two-dimensional
lattice of choice. And the cross section of the through-holes 2 is not limited to
being round but may be other shapes; or the cross-sectional form may be varied along
the slab thickness.
[0044] As given in the foregoing the present invention affords, in 2D photonic crystals,
cavities in which the
Q factor is heightened, and by combining a cavity of that sort with a waveguide, furthermore
makes available channel add/drop filters having high wavelength resolution.
[0045] Only selected embodiments have been chosen to illustrate the present invention. To
those skilled in the art, however, it will be apparent from the foregoing disclosure
that various changes and modifications can be made herein without departing from the
scope of the invention.
[0046] Furthermore, the foregoing description of the embodiments according to the present
invention is provided for illustration only, and not for limiting the invention.